Few places still used hardcoded limit for maximum XML size for commands
that accept XML files. The hardcoded limits ranged from 8k to 1M. Use
VSH_MAX_XML_FILE to express this limit in a unified way. This will bump
the limit for the commands that used hardcoded string lengths to 10M.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1152427